Two-way radio messaging systems, also known as acknowledge-back paging systems, utilize low-power transmitters in the portable subscriber units to transmit messages and acknowledgments to the base receivers of the system using an inbound channel. When a portable subscriber unit is being transported in a rapidly moving vehicle, multipath fading decreases the reliability of the inbound and outbound channel transmissions. Fading can be particularly problematic for the inbound channel, which typically utilizes messages that are too short to benefit much from well-known reliability enhancing techniques such as code interleaving.
In an exemplary two-way messaging system utilizing a 4800 bit per second inbound transmission rate, approximately 19 dB higher signal margin was determined to be required for a subscriber unit moving at 45 miles (73 kilometers) per hour, as compared to a motionless subscriber unit, to achieve a packet error rate of 1 percent. A system designed with a gain margin 19 dB higher than required for static reception would not be economically feasible. One possible solution would be to repeat all inbound messages. That solution, however, would adversely impact inbound channel throughput.
Thus, what is needed is a method and apparatus that can improve the reliability of the inbound channel transmissions. Preferably the method and apparatus will generate a minimum impact on the throughput of the inbound channel.